It has long been the practice in the building industry to employ double-glazed (or multiple-glazed) units as part of the wall of large buildings, such as office towers. While such units may be transparent, when they are used as windows, it is becoming increasingly popular to employ light diffusing units as part of the wall structure in order to increase the amount of natural light entering the building and thus decrease the artificial lighting requirements.
The glass industry uses sealed insulated glass units as a standard building block for windows and curtainwall. These are made up of two parallel glass panes, known in the industry as lites, which are separated by a spacer and sealed by a bead of sealant, typically silicone or hot-melt butyl rubber around the perimeter. These units are filled with air or inert gas which expands and contracts as the units are heated or cooled. The resultant pressure changes displace the lites of glass, and give the unit a concave or convex distortion. In normal operation temperature cycling and resultant distortion occurs on a daily basis.
Energy conservation in buildings is of prime importance. The thermal insulating properties of conventional glazing units are determined by the gap. Insulation value increases in proportion to gap because more air results in less conductive/convective heat transfer. Radiation, which typically accounts for slightly less than 50% of heat transfer, is not affected by a thicker gap, and therefore the proportionality constant is less than 1. The increase in insulation only occurs until the gap has a thickness of about ⅝″, at which point convective movement of air increases heat transfer at a rate that cancels any increase in insulation.
One material that has a number of attractive properties for use in glazing applications is silica aerogel. One such silica aerogel is sold under the name Nanogel™ by Cabot Corporation. This is a sparse silica matrix with a very high percentage (95% or more) void fraction. It is typically made by creating a silica alcogel (silica gel with alcohol as the liquid rather than water) and then removing the alcohol. This must be done at supercritical conditions in order to avoid creating surface tension effects which would collapse the gel into a denser material (non-supercritical fluid removal would create the common dehydrated silica gel particles which are used as dessicants, rather than aerogel).
silica aerogel is made of silica, it is as permanent and colorfast as glass itself. It is also one of the best insulating materials known (this is a function of the thermal infrared radiation absorption abilities of silica and the ultrafine (<50 nm scale) structure. Because silica does not absorb visible light, and the physical inhomogeneities of the gel structure are much less than a wavelength of light (50 nm vs 500 nm wavelength for green light), silica aerogel can be highly transparent. Being 95% void, silica aerogel has an index of refraction that is very close to air, so there is little surface reflection or refraction and therefore granular material can effectively transmit light without scattering.
Despite the promise, silica aerogel has a number of challenges. First, despite having been discovered a century ago, it is very difficult to make monolithic aerogel that is perfect enough to work as a component in a vision window, nor is it known how to make monolithic aerogel cost-effectively. As a result silica aerogel has been relegated to daylighting applications. Several manufacturers make plastic glazing products or rolled glass, where it is silica areogel is used as an insulating fill.
It would be desirable to improve the insulating properties without a corresponding increase in gap size.